International audienceGelatine has often been used as an analogue material to model the propagation of magma-filled fractures in the Earth's brittle and elastic crust. Despite this, there are few studies of the elastic properties of gelatine and how these evolve with time. This important information is required to ensure proper scaling of experiments using gelatine. Gelatine is a viscoelastic material, but at cool temperatures (Tr ~ 5-10 °C) it is in the solid 'gel' state where the elastic behaviour dominates and the viscous component is negligible over short to moderate timescales. We present results from a series of experiments on up to 30 litres of maximum 30 wt.% pigskin gelatine mixtures that document in detail how the elastic properties evolve with time, as a function of the volume used and gel concentration (Cgel). Gelatine's fracture toughness is investigated by measuring the pressure required to propagate a pre-existing crack. In the gel-state, gelatine's Young's modulus can be calculated by measuring the deflection to the free-surface caused by an applied load. The load's geometry can affect the Young's modulus measurement; our results show its diameter needs to be ≲ 10% of both the container diameter and the gelatine thickness (Hgel) for side-wall and base effects to be ignored. Gelatine's Young's modulus increases exponentially with time, reaching a plateau (E∞) after several hours curing. E∞ depends linearly on Cgel, while Tr, Hgel and the gelatine's thermal diffusivity control the time required to reach this value. Gelatine's fracture toughness follows the same relationship as ideal elastic-brittle solids with a calculated surface energy γs = 1.0 ± 0.2 J m− 2. Scaling laws for gelatine as a crustal analogue intruded by magma (dykes or sills) show that mixtures of 2-5 wt.% gelatine cured at ~ 5-10 °C ensure the experiments are geometrically, kinematically and dynamically scaled

[1] Sills could potentially form as a result of dykes modifying their trajectory in response to remote tectonic compression. Here, we use analogue experiments to investigate how a buoyant vertical dyke adjusts its trajectory to a compressive remote stress to form a sill, and over which vertical distance this sill formation does occur. Our investigation is restricted to an intrusion propagating through a homogeneous solid, which enables us to identify the characteristic length-scale over which a dyke responds to remote stress compression, independently of the presence of crustal layers. The experiments involve the injection of air in a gelatine solid that experiences lateral deviatoric compression. The response of the buoyant air crack to the compressive stress in not instantaneous but operates over some distance. An important observation is that some cracks reach the surface despite the compressive environment. Dyke-to-sill rotation occurs only for large compressive stress or small effective buoyancy. Dimensional analysis shows that the length-scale over which this rotation takes place increases exponentially with the ratio of crack effective buoyancy to horizontal compressive stress. Up-scaled to geological conditions, our analysis indicates that a dyke-to-sill transition in response to tectonic compression in homogeneous rocks cannot occur over less than two hundred meters and would need several kilometers in most cases. This is typically greater than the average thickness of lithological units, which supports the idea that crustal heterogeneities play an important role in determining the fate of dykes and in controlling where sills could form.

International audienceA consensus has emerged in recent years from a variety of geoscientific disciplines that extension during continental rifting is achieved only partly by plate stretching: dyke intrusion also plays an important role. Magma intrusion can accommodate extension at lower yield stresses than are required to extend thick, strong, unmodified continental lithosphere mechanically, thereby aiding the breakup process. Dyke intrusion is also expected to heat and thereby weaken the plate, but the spatial extent of heating and the effect of different rates of magmatic extension on the timescales over which heating occurs are poorly understood. To address this issue, a numerical solution to the heat-flow equation is developed here to quantify the thermal effects of dyke intrusion on the continental crust during rifting. The thermal models are benchmarked against a priori constraints on crustal structure and dyke intrusion episodes in Ethiopia. Finite difference models demonstrate that magmatic extension rate exerts a first-order control on the crustal thermal structure. Once dyke intrusion supersedes faulting and stretching as the principal extensional mechanism the crust will heat and weaken rapidly (less than 1 Ma).In the Main Ethiopian Rift (MER), the majority of present-day extension is focused on ∼20 km-wide Quaternary-Recent axial magmatic segments that are mostly seismogenic to mid-crustal depths and show P-wave seismic velocities characteristic of heavily intruded continental crust. When reviewed in light of our models, these observations require that no more than half of the MER's extension since ∼2 Ma has been achieved by dyke intrusion. Magmatic heating and weakening of the crust would have rendered it aseismic if dyke intrusion accounted for the entire 6 mm/yr extension rate. In the older, faster extending (16 mm/yr) Red Sea rift (RSR) in Afar, dyke intrusion is expected to have had a more dramatic impact on crustal rheology. Accordingly, effective elastic plate thickness and Moho depth in the Danakil region of northernmost Afar are markedly reduced and seismicity is shallower than in the MER. Thermally driven variations in crustal rheology over time in response to dyke intrusion thus play an important role in the development of continent–ocean transition

International audienceWe document the shape of dikes from well exposed field locations in the Isle of Rum, Scotland,14 and Helam Mine, South Africa. The basaltic Rum dikes crop out on a smaller scale than the15 Helam kimberlite dikes and have a smaller length to thickness ratio (~100:1 Isle of Rum,16 ~1000:1 Helam Mine). We compare dike thickness field measurements with the geometry17 predicted by elastic theory, finding best-fit models to estimate magma overpressure and regional18 stress gradients at the time of dike emplacement. Most of the dike shapes fit poorly with elastic19 theory, being too thick at the dike ends and too narrow in the middle. Our calculated20 overpressures and stress gradients are much larger than independent estimates based on rock21 strength. Dike shape can be explained by a combination of host rock inelastic deformation and22 magma chilling at the dike’s tapering edges preventing its closure as magma pressure declines23 during emplacement. The permanent wedging of the dike edges due to chilling has implications24 for crustal magma transport and strain response in the crust due to dike emplacemen

(2013) 'The formation of columnar joints produced by cooling in basalt at Staa, Scotland.', Bulletin of volcanology., 75 (6). p. 715.Further information on publisher's website:http://dx.doi.org/10.1007/s00445-013-0715-4Publisher's copyright statement:The nal publication is available at Springer via http://dx.doi.org/10.1007/s00445-013-0715-4.Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details.